SELECT 
  linked_promotion_id 
FROM 
  ushop_product_bundles

Query time 0.00025

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.92"
    },
    "table": {
      "table_name": "ushop_product_bundles",
      "access_type": "ALL",
      "rows_examined_per_scan": 4,
      "rows_produced_per_join": 4,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "0.52",
        "eval_cost": "0.40",
        "prefix_cost": "0.92",
        "data_read_per_join": "160"
      },
      "used_columns": [
        "linked_promotion_id"
      ]
    }
  }
}